The best recommendations are Nomex, on top of cotton undershirt and cotton long-johns as a minimum whatever the weather.
Spot on and that is what the military always say. Sadly, in the heat of the summer (or in the desert) cotton longjohns can often be more of a hazard because of body-temp-overheat-induced thermal stress. One has to weigh up the risks but I am as guilty as the next man and often did away with that extra layer on hot days. The only aircraft in which I didn't do this was the Bulldog (Scottish Aviation version), where there was a fuel feed directly to the fuel pressure guage which was just above one's knees. I sweated buckets but thought the risks too great not to wear longjohns.
I tried to get a major UK airline to consider sanctioning the wearing of cotton trousers and a 99% cotton pilot shirt (Brookes Brothers, I think and so quite pricey - and they no longer make them). I was told to shut up by my line-manager who made a point of taking it no further and even criticised me for wearing the cotton trousers ('against company policy' he said even when it wasn't); his attitude was the straw that broke the camel's back, so I left! Not that new company will pay for it either but at least they listen to safety concerns but sadly, it is too costly.
Read "9 minutes 20 seconds" if you want a reason to consider longjohns, a t-shirt and leather gloves (
Amazon.com: Nine Minutes, Twenty Seconds: A True Story of Tragedy and Triumph (9780609810163): Gary M. Pomerantz: Books)